Critical ion thruster component that affects operation and service life is the grids assembly, and the hot gap variation of the grids directly determines the erosion process of the grids. In order to obtain the hot gap variation of the triple grid for a 30cm diameter ion thruster under 5kW work condition, finite element analysis is used to calculate the distribution of temperature and thermal deformation, then a deflection measurement experiment is carried out to verify the simulations. The results show that the temperature change of the screen grid is the fastest for the lowest heat capacity and the highest deposition energy among the three grids. The hot gap between the center area of the screen grid and the accelerator grid is decreased from 0.95 mm to 0.45 mm in the first 5 minutes. Meanwhile, the central gap is decreased from 0.85 mm to 0.42 mm between the accelerator grid and the decelerator grid after the thruster works about 2000s. The thermal deformation of the decelerator grid presents a “trapezoid” shape. On the contrary, that of the accelerator grid presents a “parabolic” shape. The results of simulation and measurement of temperature in the edge of the screen grid are 338 ℃ and 321 ℃ , respectively. The thermal displacement test results show that the hot gap in the center area between the screen grid and the accelerator grid, the accelerator grid and the decelerator grid are decreased from 0.95 mm to 0.48 mm, and 0.85 mm to 0.46 mm, respectively. The simulation results are consistent with test results, and the errors are considered mainly come from the structural equivalent model and which is calculated to be less than 10%.